[tmva][sofie] Add README for SOFIE parsers

[Shlok-Saxena]

This Pull request:

Adds a README to the root/tmva/sofie_parsers directory describing the purpose, scope, basic usage, and internal structure of the SOFIE parser infrastructure. The documentation is intended to be descriptive and does not introduce any functional changes.

The documentation is intended to complement the existing SOFIE README with parser-specific information for both users and contributors, and can be extended in the further if needed.

Changes or fixes:

• Add a comprehensive README for the SOFIE parsers
• Document supported model formats and parser responsibilities
• Describe ONNX operator support and validation workflow
• Add developer-oriented notes on parser architecture and extension

Checklist:

• tested changes locally
• updated the docs (if necessary)